When major executives sell large numbers of shares, these sales are often very public. Generally, massive stock dumps are a sign that the executive feels the value of the company has peaked, and is likely to drop. In other words, every large sale of stock is implicitly a public announcement of lack of confidence in the company by the executive.
Legally, executives have a fiduciary duty to their companies, in that they are expected not to take actions harming the company and its value in any major way. Considering a large sale of stock by a company executive tends to lower public confidence in a company, and thus lower the value of the company, could such a sale itself be considered a breach of fiduciary duty?